Solvent-based and water-based carbon nanotube inks with removable additives

ABSTRACT

In accordance with some embodiments, compositions and methods for forming solvent-based and water-based carbon nanotubes inks with removable additives are provided. In some embodiments, an ink composition is provided that includes a plurality of carbon nanotubes, a solvent, and a triazole-based removable additive, where the plurality of carbon nanotubes are dispersed within the solvent and wherein the triazole-based removable additive stabilizes the plurality of carbon nanotubes that are dispersed in the solvent.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/855,640 filed Aug. 12, 2010, which claims the benefit ofU.S. Provisional Patent Application No. 61/234,203, filed Aug. 14, 2009,which are hereby incorporated by reference herein in their entireties.

TECHNICAL FIELD

The disclosed subject matter relates to the formation of dispersions orinks of carbon nanotubes. More particularly, the disclosed subjectmatter relates to the formation of surfactant-free carbon nanotube inksin water and solvent media obtained with the use of removable additives.

BACKGROUND

Most applications of single-walled carbon nanotubes (SWCNT),double-walled carbon nanotubes (DWCNT), and multi-walled carbonnanotubes (MWCNT) often require that they are available in the form ofdispersions in a purified form in a suitable solvent system. These typesof carbon nanotubes are generically described as carbon nanotubes (CNT)unless otherwise indicated.

As produced raw carbon nanotube soots generally include materialimpurities (extraneous impurities), such as transition metal catalysts,graphitic carbons, amorphous carbon nanoparticles, fullerenes, carbononions, and polycyclic aromatic hydrocarbons along with the desiredcarbon nanotube products. The nature and degree of the electronicimpurities in a given raw material can depend on the method ofsynthesis, such as, for example, laser, arc, High-Pressure CarbonMonoxide Conversion (HiPco), chemical vapor deposition (CVD), orcombustion.

Known purification protocols generally involve steps of generic unitoperations like pre-oxidation, acid reflux, mechanical mixing,ultrasonication, filtration, neutralization, and centrifugation.Selecting a suitable combination depends upon the method of productionof the carbon nanotubes and the specific impurity targeted. As shownbelow. Table 1 provides an exemplary list of the dominant impurities indifferent nanotube samples and unit operations employed in theirpurification.

TABLE 1 Catalyst Dominant metal carbon Unit Operations SI No Tube Typeimpurities impurities Employed Year Reference 1 Laser Co, Ni GraphiticHNO₃ reflux, 1998 Rinzler et al., neutralization, Appl. Phys.centrifugation, cross A 67, 29-37 flow filtration (1998) 2 Laser Co, NiGraphitic gas oxidation, HCl 2001 Chiang et al., washing J. Phys. Chem.B 105, 8297 (2001) 3 Arc Ni, Y Graphitic microwave exposure, 2002Harutyunyan HCl washing et al., J. Phys. Chem. B 106, 8671 (2002) 4 CVDCo, Fe, Ni Amorphous air oxidation, HF 1999 Colomer et supported washingal., Synthetic on zeolites Metals 103, 2482 (1999) 5 HiPCO FeFullerenes, wet air oxidation, HCl 2002 Sivarajan et Amorphous washingand fluorinated al., J. Phys. extraction of fullerenic Chem. B 107,impurities 1361 (2003) 6 HiPCO Fe Fullerenes, H₂SO₄ + HNO₃ 2004Wiltshire et Amorphous sonication al., Chemical Physics Letters 386, 239(2004) 7 HiPCO Fe Fullerenes, one pot HCl + H₂O₂ 2007 Wang et al.,Amorphous washing J. Phys, Chem. B 111, 1249-1252 (2007)

A. G. Rinzler et al., “Large-scale purification of single-walled carbonnanotubes: process, product, and characterization,” Appl. Phys. A 67,29-37 (1998) describes a large-scale purification approach for purifyingcarbon nanotubes employing a sequence of steps including, for example,nitric acid reflux, neutralization, centrifugation, and cross-flowfiltration as essential steps to purify single-walled carbon nanotubes.

Extraneous impurities, such as catalyst metal particles, fullereniccarbon, amorphous carbon, graphitic carbon, and carbon onions, arepresent to different degrees in as prepared raw carbon nanotube samples.Oxidative chemical treatments as part of the purification protocol andmultiple acid treatments as part of the typical purification processesresult in reasonably clean carbon nanotubes (<0.5 wt % impurities).However, since the intrinsic electrical conductivity arises from thedelocalized π electrons of the SWCNT for a SWCNT of a given length anddiameter, an aggressive chemical purification or side-wallderivatization during the purification process drains the π electrons ofthe individual SWCNT. Such a loss of conductive electrons leads to adrastic fall in the single tube electrical conductance as well as theelimination of the inter-band optical transitions arising from the vanHove singularities. Accordingly, for many applications, especiallyapplications requiring a combination of optical and electricalproperties retaining the electronic structure of the CNT substantiallyintact is an important aspect in the formation of SWCNT inks.

There are numerous approaches that form stable dispersions of carbonnanotubes in water with the use of anionic, cationic, or non-ionicsurfactants. These surfactants form a monolayer coating on the surfaceof the CNT in the dispersed form either as individuals or as thinbundles. There are also widely reported approaches that use ionic orneutral polymer molecules for solubilizing carbon nanotubes in a watermedium. Known examples are, among others, polystyrene sulfonate,polyvinyl pyrrolidinone, polyethylene oxide (PEO), polypropylene oxide(PPO), and tri-block copolymers of PEO-PPO-PEO. However, when thin filmsof CNT networks are formed on solid substrates from such dispersions,most of the surfactants or the polymers remain as part of the carbonnanotube films/network as a coating on the carbon nanotubes and remainthere even after treatments at elevated temperatures. Presence of suchsurface impurities affect the electronic properties of the carbonnanotube network—e.g., reducing the electrical conductivity of thenetwork.

Another approach for forming carbon nanotube dispersions or inks inorganic solvents is to chemically derivatize them. For example, Haddonet al., U.S. Pat. No. 6,331,262, describes an approach that involves endfunctionalization employing carboxylation followed by acid-chlorideformation followed by the formation of amide linkage by reacting with along chain amine. However, the resulting solutions in organic solventsdid not show the characteristic absorption features in the UV-Visiblerange, thereby suggesting that the delocalized π electrons have beendrained completely or significantly. In addition, it should be notedthat the electrical properties of the functionalized carbon nanotubeswere not reported.

Huang et al., U.S. Patent Publication No. 2006/0124028 A1, describescarbon nanotube ink compositions in an aqueous medium designed forinkjet printing, which were obtained by a chemical reaction involving anazo compound and carboxylated single-walled carbon nanotubes. Thisapproach focuses on the ink-jet printability of the dispersion or inkrather than the intrinsic properties of the CNT altered by theazo-functionalization.

Carbon nanotube inks prepared, despite these prior art approaches,especially for SWCNT suffer from one or more of the followinglimitations:

a) Loss of inter-band optical transitions indicating a significantmodification of the electronic structure of the simile-walled carbonnanotubes or electronic defects; and/or

b) Surfactant or polymeric dispersal aid residues that are not removablefrom the solid film when such inks are used to form carbon nanotubenetworks or films.

There is a need in the art for approaches that provide the formation ofstable carbon nanotube inks in water or organic solvent media in which(a) the SWCNT have not lost their inter-band optical transitionssignifying an intact electronic structure and (b) the dispersal aidsthat are used to stabilize the SWCNT do not leave non-volatile residuein the solid products such as films formed from such inks. Accordingly,it is desirable to provide solvent-based and water-based carbon nanotubeinks that overcome these and other deficiencies of the prior art.

For example, in some embodiments, a dispersal aid system that isnon-ionic, molecular in nature, conserves the electronic structure ofthe SWCNT as evidenced by the inter-band optical transitions and thatuses dispersal aids that can be completely removed from the carbonnanotube network or films that are formed using the CNT ink.

SUMMARY

Applications of single-walled carbon nanotubes (SWCNT), double-walledcarbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT)generally require carbon nanotubes in the form of dispersions insuitable solvent, systems. Raw carbon nanotube soots generally includematerial impurities such as transition metal catalysts, graphiticcarbons, amorphous carbon nanoparticles, fullerenes, carbon onions,polycyclic aromatics along with desired carbon nanotube products.

The nature and degree of the impurities in a given raw material dependson the method of synthesis, such as, for example, laser, arc,High-Pressure Carbon Monoxide Conversion (HiPco), chemical vapordeposition (CVD), or combustion methods. Currently availablepurification processes and systems generally involve generic unitoperations, such as pre-oxidation, acid reflux, mechanical mixing,ultrasonication, filtration, neutralization, and centrifugation.

For example, single-walled carbon nanotubes (SWCNT) can be producedusing a premixed combustion of carbon-Containing fuels, such ashydrocarbons, including methane, natural gas, or alcohols, while a metalcatalyst precursor (such as iron pentacarbonyl, ferrrocene, or a metalsalt solution) is added continuously to the fresh gas mixture.Characteristics of the SWCNT formed, such as length, can be controlledby process parameters (e.g., pressure, inert gas dilution, temperature,fresh gas velocity, residence time, etc.) By-products are reactionproducts of the catalyst precursor, for example, iron or iron oxides(particularly Fe₂O₃) and carbonaceous material other than SWCNT, such aspolycyclic aromatic hydrocarbons (PAH).

It should be noted that, although the present invention is generallydescribed in connection with the purification and dispersion of flamesynthesized carbon nanotubes, this is merely illustrative. The disclosedsubject matter can provide, among other possible advantages andbeneficial features, methods, techniques, apparatuses, systems, and/ormethods of manufacture that can be used for the purification andformation of water-based or solvent-based suspensions of carbonnanotubes of all types.

In some embodiments, small molecular additives, such asdiethylenetriamine (DETA) and diisopropylethylamine (DIPEA or Hunig'sbase), can be used as stabilizing additives that disperse single-walledcarbon nanotubes without the elimination of the inter-band opticaltransitions or viscosity adjustment agents.

It should be noted that Hunig's base (DIPEA) is not soluble in water. Insome embodiments, the present invention describes a method for making awater-based dispersion of single-walled carbon nanotubes employingamines that are not necessarily soluble in water.

In some embodiments, the present invention describes the formation ofsolvent-based CNT inks. For example, solvent-based CNT inks can beformed using N-methylpyrrolidinone (NMP) as the solvent andpolypropylene carbonate oligomer as an additive that can be completelyremoved. It should be noted that the additive can act a stabilizingagent, a viscosity adjustment agent, or any suitable combinationthereof.

Alternatively, in some embodiments, small molecular additives, such as atriazole-based compound, can be used as stabilizing additives thatdisperse single-walled carbon nanotubes without the elimination of theinter-band optical transitions or viscosity adjustment agents. In a moreparticular embodiment, the triazole-based compound is 1,2,4-Triazole.

It should be noted that, in some embodiments, the triazole-basedadditive can be unsubstituted 1,2,4-Triazole. For example, as shown inthe chemical formula below, each of R₁, R₂, and R₃ can be hydrogen.

Alternatively, the triazole-based additive can be substituted1,2,4-Triazole. Substituted 1,2,4-Triazole can be used as an additive inwater-based solvents and also in non-aqueous solvents based on theselected substituents. That is, R₁, R₂, and R₃ can be selected in orderto achieve solubility in targeted solvents. For example, two or threesubstituting groups can be identical. In another example, one or twogroups can be hydrogen. In yet another example, R₁, R₂ and R₃ (sometimesreferred to herein as “R”) can be straight-chain or branched or cyclicalkyl chains (C₁ to C₂₀) which can be unsubstituted, monosubstituted, orpoly substituted. Substituents can be selected from at least one of thefollowing: OH, OR, CO₂R, OOCR, SO₃H, X (where X is F, Cl, Br, NO₂ and/orCN), SO₂X, COX, NH₂, NR₂, NR₃ ⁺, substituted or unsubstituted benzyl(CH₂C₆H₅), substituted or unsubstituted phenyl, thiophene-radicals,H₂PO₄, and mixtures thereof. It should be noted that, in someembodiments, one or more CH₂ groups (including the one adjacent totriazole and establishing the link) can be replaced by one of thefollowing units: O, CO, NH, NHR⁺, SO₂, a cyclic alkyl, a substituted orunsubstituted aromatic ring containing only carbon or carbon andheteroatoms, the latter including nitrogen, sulfur, or oxygen.

As used herein, an “optionally substituted” group generally refers tofunctional groups that can be monosubstituted, polysubstituted, orunsubstituted by additional functional groups. When a group isunsubstituted by an additional group, it can be referred to as a groupname, for example, alkyl. When a group is substituted with additionalfunctional groups, it can be more generally referred to as substitutedalkyl. As also used herein, “alkyl” generally refers to straight chainor branched or cyclic alkyl groups having from 1 to 20 carbon atoms (C₁to C₂₀) (or from 1 to 15 carbon atoms, etc.).

In some embodiments, the additives described herein (e.g., stabilizingagents, dispersal aids, or viscosity adjustment agents) can also be usedfor the dispersion of other carbonaceous nanostructures, such as, forexample, graphene, fullerenes like C₆₀ and C₇₀, shortened nanotubes(e.g., fullerene pipes), and nanofibers of any of these compounds inwater and/or organic solvent media. In addition, these additives can beused for chemical derivatives of all carbonaceous nanostructuresincluding, for example, single-walled carbon nanotubes, double-walledcarbon nanotubes, multi-walled carbon nanotubes, graphene, fullerenes,shorted carbon nanotubes, and nanofibers.

In accordance with some embodiments of the present invention, an inkcomposition is provided, the ink composition comprising: a plurality ofcarbon nanotubes; solvent; and a triazole-based removable additive thatstabilizes the plurality of carbon nanotubes in the solvent.

In some embodiments, the plurality of carbon nanotubes are single-walledcarbon nanotubes.

In some embodiments, the plurality of carbon nanotubes are a mixture ofmetallic and semiconducting single-walled carbon nanotubes.

In some embodiments, the plurality of carbon nanotubes are enriched inmetallic single-walled carbon nanotubes.

In some embodiments, the plurality of carbon nanotubes are metallicsingle-walled carbon nanotubes.

In some embodiments, the plurality of carbon nanotubes are enriched insemiconducting single-walled carbon nanotubes.

In some embodiments, the plurality of carbon nanotubes are single-walledcarbon nanotubes with a specific chirality.

In some embodiments, the solvent is one of: water, N-methylpyrrolidinone(NMP), propylene glycol monomethyl ether acetate (PGMEA), methyl ethylketone (MEK), and methyl isopropyl ketone.

In some embodiments, the triazole-based removable additive is selectedto act as a dispersal agent and a stabilization agent.

In some embodiments, the removable additive is selected to adjustviscosity of the ink based at least in part on molecular weight of theremovable additive.

In some embodiments, the triazole-based removable additive is a1,2,4-triazole compound having a chemical formula:

In some embodiments, each of R₁, R₂, and R₃ is hydrogen. In someembodiments, at least one of and R₁, R₂, and R₃ is hydrogen. In someembodiments, at least one of R₁, R₂, and R₃ is an C₁-C₂₀ alkyl group.

In some embodiments, the C₁-C₂₀ alkyl group is optionally substitutedwith at least one substituent selected from one of: OH, OR, CO₂R, OOCR,SO₃H, X, SO₂X, COX, NH₂, NR₂, NR₃ ⁺, optionally substituted benzyl,optionally substituted phenyl, thiophene radicals, and mixtures thereof,wherein R is the C₁-C₂₀ alkyl group and X is one of: F, Cl, Br, NO₂, andCN.

In some embodiments, one or more CH₂ groups in the C₁-C₂₀ alkyl group isoptionally substituted with at least one substituent selected from oneof: O, CO, NH, NHR⁺, a cyclic alkyl, an optionally substituted aromaticring containing carbon, an optionally substituted aromatic ringcontaining carbon and heteroatoms includes at least one of nitrogen,sulfur, and oxygen.

In some embodiments, the triazole-based removable additive is optionallysubstituted 1,2,4-Triazole and wherein one or more functional groups inthe 1,2,4-Triazole are optionally substituted with at least onesubstituent that is selected based on the solvent.

In some embodiments, the triazole-based removable additive is removedfrom the ink composition by thermal annealing.

In accordance with some embodiments, a method of preparing an inkcomposition is provided, the method comprising: reacting a plurality ofcarbon nanotubes, a triazole-based removable additive, and a solvent,wherein the plurality of carbon nanotubes are dispersed within thesolvent and wherein the triazole-based removable additive stabilizes theplurality of carbon nanotubes that are dispersed in the solvent.

In some embodiments, the plurality of carbon nanotubes are single-walledcarbon nanotubes.

In some embodiments, the plurality of carbon nanotubes are a mixture ofmeta sc and semiconducting single-walled carbon nanotubes.

In some embodiments, the plurality of carbon nanotubes are enriched inmetallic single-walled carbon nanotubes.

In some embodiments, the plurality of carbon nanotubes are metallicsingle-walled carbon nanotubes.

In some embodiments, the plurality of carbon nanotubes are enriched insemiconducting single-walled carbon nanotubes.

In some embodiments, the plurality of carbon nanotubes are single-walledcarbon nanotubes with a specific chirality.

In some embodiments, the solvent is one of: water, N-methylpyrrolidinone(NMP), propylene glycol monomethyl ether acetate (PGMEA), methyl ethylketone (MEK), and methyl isopropyl ketone.

In some embodiments, the triazole-based removable additive is a1,2,4-triazole compound having a chemical formula:

In some embodiments, the triazole-based removable additive isunsubstituted 1,2,4-Triazole. Alternatively, the triazole-basedremovable additive is substituted 1,2,4-Triazole, wherein one or moresubstituents are selected based on the solvent.

In some embodiments, each of R₁, R₂, and R₃ in the 1,2,4-triazolecompound is hydrogen. In some embodiments, at least one of R₁, R₂, andR₃ in the 1,2,4-triazole compound is hydrogen.

In some embodiments, at least one of R₁, R₂, and R₃ in the1,2,4-triazole compound is an C₁-C₂₀ alkyl group. In some embodiments,the C₁-C₂₀ alkyl group is optionally substituted with at least onesubstituent selected from one of: OH, OR, CO₂R, OOCR, SO₃H, X, SO₂X,COX, NH₂, NR₂, NR₃ ⁺ optionally substituted benzyl, optionallysubstituted phenyl, thiophene radicals, H₂PO₄, and mixtures thereof,wherein R is the C₁-C₂₀ alkyl group and X is one of: F, Cl, Br, NO₂, andCN. In some embodiments, one or more CH₂ groups in the C₁-C₂₀ alkylgroup is optionally substituted with at least one substituent selected,from one of O, CO, NH, NHR⁺, a cyclic alkyl, an optionally substitutedaromatic ring containing carbon, an optionally substituted aromatic ringcontaining carbon and heteroatoms includes at least one of nitrogen,sulfur, and oxygen.

In some embodiments, the method further comprises providing theplurality of carbon nanotubes in the form of a wet paste to a solutionthat includes the triazole-based removable additive and the solvent.

In some embodiments, the method further comprises stabilizing theplurality of carbon nanotubes by providing the triazole-based removableadditive to a solution that includes the plurality of carbon nanotubesand the solvent.

In some embodiments, the method further comprises stabilizing theplurality of carbon nanotubes by applying the triazole-based removableadditive to the plurality of carbon nanotubes prior to dispersing theplurality of carbon nanotubes in the solvent and providing the solventto the plurality of carbon nanotubes and the triazole-based removableadditive.

In some embodiments, the method further comprises applying the inkcomposition to a substrate and removing a substantial portion of thetriazole-based removable additive by thermal annealing, wherein thetriazole-based removable additive is removed after applying the inkcomposition to a substrate. The substrate can be a glass substrate, aplastic substrate, and/or a sapphire substrate.

In some embodiments, the triazole-based removable additive is removedafter applying the ink composition to the substrate. For example, asubstantial portion of the triazole-based removable additive can beremoved after applying the ink composition to the substrate by thermalannealing. The triazole-based removable additive is selected such thatthe triazole-based removable additive can be removed from the inkcomposition by at least 90% by thermal annealing at a temperature lowerthan about 250° C.

In some embodiments, the method further comprises purifying theplurality of carbon nanotubes prior to adding the triazole-basedremovable additive and the water-based solvent. In some embodiments, theplurality of carbon nanotubes are purified by washing the plurality ofcarbon nanotubes in a solution of ammonium hydroxide.

In some embodiments, the method further comprises purifying a mixtureincluding the plurality of carbon nanotubes, the triazole-basedremovable additive, and the water-based solvent by reducing impuritiesusing centrifugation. In some embodiments, the centrifugation reducesamorphous carbon impurities. In some embodiments, a first portion of thecentrifuged mixture is stored as the ink composition and a secondportion of the centrifuged mixture is discarded.

In some embodiments, the method further comprises passing at least aportion of the centrifuged mixture through a filter to remove particleimpurities having a diameter greater than a given size.

In accordance with some embodiments, a method of preparing an inkcomposition is provided, the method comprising: providing a paste thatincludes a plurality of single-walled carbon nanotubes; purifying thepaste that includes plurality of single-walled carbon nanotubes in asolution of ammonium hydroxide to substantially reduce amorphous carbonimpurities; forming a mixture by adding a 1,2,4-triazole compound and awater-based solvent to the purified paste that includes the plurality ofsingle-walled carbon nanotubes, wherein the plurality of single-walledcarbon nanotubes are dispersed within the water-based solvent andwherein the 1,2,4-triazole compound stabilizes the plurality ofsingle-walled carbon nanotubes that are dispersed in the water-basedsolvent; and purifying the mixture by centrifugation, wherein a firstportion of the centrifuged mixture is stored as the ink composition anda second portion of the centrifuged mixture is discarded.

In some embodiments, the plurality of carbon nanotubes dispersed withinthe solvent can be separated between functionalized and unfunctionalizedcarbon nanotubes (e.g., using density gradient centrifugation orelectrophoresis). In addition, in some embodiments, the plurality ofcarbon nanotubes dispersed within the solvent can be separated betweenmetallic and semiconducting carbon nanotubes (e.g., using chemical orelectrophoresis approaches) with or without prior functionalization. Forexample, in some embodiments, the plurality of carbon nanotubes can beseparated such that at least 80% of the plurality of carbon nanotubesare semiconducting single-walled carbon nanotubes. In some embodiments,the carbon nanotube ink described herein can be enriched in eithersemiconducting or metallic single-walled carbon nanotubes in comparisonto their initial abundance in the as-produced material (e.g., oftenapproximately a 2:1 ratio of semiconducting vs. metallic correspondingto the ensemble of all theoretically existing chiralities).

The details of one or more embodiments of the subject matter describedherein are set forth in the accompanying drawings and the descriptionbelow. Other features and advantages of the subject matter describedherein will be apparent from the description and drawings, and from theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart showing a Raman spectrum of as produced carbonnanotube material in accordance with some embodiments of the presentinvention.

FIG. 2 is a chart showing an example of a thermogravimetric analysis ofa raw carbon nanotube material containing about 50% of a catalyst metalimpurity that takes into account the oxidation of initially present ironto iron oxide in accordance with some embodiments of the presentinvention.

FIG. 3 is chart showing an example of a thermogravimetric analysis of apurified carbon nanotube material in accordance with some embodiments ofthe present invention.

FIG. 4 is a process flow chart showing a method for preparingaqueous-based carbon nanotube inks in accordance with some embodimentsof the present invention.

FIG. 5 is a chart showing an ultraviolet-visible absorption spectrum ofa carbon nanotube based ink as a final product in a water-basedemploying DIPEA as the stabilizing agent, where the interband opticaltransitions indicative of an intact electronic structure of the SWCNTare marked with black arrows, in accordance with some embodiments of thepresent invention.

FIG. 6 is a chart showing an ultraviolet-visible absorption spectrum ofa carbon nanotube film deposited from the water-based ink on a 6″×4″glass substrate, where the interband optical transitions arising fromthe first van Hove transitions are shown with white arrows and thesecond van Hove transitions are shown with black arrows, in accordancewith some embodiments of the present invention.

FIG. 7 is a scanning electron microscope (SEM) image of a dense carbonnanotube network deposited on a sapphire substrate in accordance withsome embodiments of the present invention.

FIG. 8 is an exemplar schematic diagram showing the decomposablepolypropylene carbonate molecules wrap around the carbon nanotubes tosuspend them in the organic solvent in accordance with some embodimentsof the present invention.

FIG. 9 shows a thermogravimetric analysis (TGA) plot and its firstderivative of polypropylene carbonate (PPC) showing the sharpdecomposition of the polymer in accordance with some embodiments of thepresent invention.

FIG. 10 is a process flow chart showing a method for preparingsolvent-based carbon nanotube inks in accordance with some embodimentsof the present invention.

FIG. 11 is a scanning electron micrograph (SEM) image of plastic beadscoated with carbon nanotubes in accordance with some embodiments of thepresent invention.

FIG. 12 is a chart showing an ultraviolet-visible-near-infrared(UV-Vis-NIR) absorption spectrum of a single-walled carbon nanotube inkthat includes a water-based solvent and a triazole-based removableadditive in accordance with some embodiments of the present invention.

FIG. 13 is a transmission electron micrograph (TEM) image ofsingle-walled carbon nanotubes from FIG. 12 that were purified to removeamorphous carbon impurities in accordance with some embodiments of thepresent invention.

FIG. 14 is a chart showing an ultraviolet-visible-near-infrared(UV-Vis-NIR) absorption spectrum of a single-walled carbon nanotube inkthat includes a water-based solvent and a triazole-based removableadditive in accordance with some embodiments of the present invention.

DETAILED DESCRIPTION Analysis of as Produced Carbon Nanotubes

Carbon nanotubes such as those produced using the disclosed subjectmatter can be analyzed and characterized using, among other possibleoptions. Raman spectroscopy and/or thermogravimetric analysis (TGA).

Resonance Raman spectroscopy provides a fast and selective method forthe identification and first characterization of SWCNTs. Majoridentifiable absorption features include the radial breathing mode(RBM), the tangential mode (G-band), and the disorder-induced band(D-band). RBM, which usually appears between 120 cm⁻¹<ω^(RBM)<270 cm⁻¹,generally corresponds to the atomic vibration of the carbon atoms in theradial direction. Direct correlations with SWCNT diameters have beengenerally established. The tangential mode or G-band, a characteristicmulti-peak feature typically occurring around 1580 cm⁻¹, corresponds toatomic displacements along the tube axis as well as the circumferentialdirection. Simultaneous observation of RBM and g-band provides strongevidence for the presence of SWCNT. The d-band, occurring around 1350cm⁻¹, reflects the presence of impurities or other symmetry-breakingdefects, such as amorphous carbon.

For the example described below, material synthesized and collectedunder well defined conditions was investigated with a Dimension-P2.Raman system (Lambda Solutions, Waltham, Mass.) using an excitingwavelength of 784.87 nm and approximately 10 mW power. A laser beam witha diameter of about 200 micrometers was directed without microscope tothe samples from a distance of about 1 cm. Generally, a 5 secondexposure time and integration over 5 spectra were applied. Peak heightsof the RBM and G-bands were optimized by fine-timing the distancebetween the laser probe and the samples. Samples with strong RBM andweak D-bands as well as high G- to D-band ratios have been considered asbeing at or close to the optimized conditions and submitted to furtheranalysis, such as scanning electron microscopy (SEM) and TGA, the latterallowing, for a quantitative assessment of SWCNT abundance in givensamples.

As shown in the Raman spectrum of FIG. 1, the D-hand was found to bebarely detectable while RBM peaks at 229.6 cm⁻¹ and 265.5 cm⁻¹ wereidentified. Using the relationship ω^(RBM)=234/d_(i)+10 cm⁻¹, assuggested by Milnera et al., “Periodic Resonance Excitation andIntertube Interaction from Quasicontinuous Distributed Helicities inSingle-Wall Carbon Nanotubes,” Phys. Rev. Lett. 84, 1324-1327 (2000),for bundles of SWCNT, these peaks correspond to diameters of about 1.07and about 0.92 nm, respectively. However, due to the strong dependenceof Raman intensities on the resonance energies of the SWCNT present,such a diameter distribution generally reflects only SWCNT resonating at784.87 nm and may not be representative for the investigated sample. Forexample, a Raman spectrum of similar material measured at 647 nm gives asignificantly different picture: RBM peaks corresponding to 1.30, 0.98and 0.87 nm have also been identified.

It should be noted that, as shown in FIG. 1, the shape of the G-bandoccurring in the 1500-1605 cm⁻¹ range corresponds to tangentialvibrations indicating the presence of both conducting SWCNTs andsemi-conducting SWCNTs. It should further be noted that, as also shownin FIG. 1, the weakness of the peak near 1350 cm⁻¹ indicates that aninsignificant low level of impurities or other symmetry-breaking defectsis present.

The purity of bulk SWCNT samples can be determined usingthermogravimetric analysis (TGA) under air, for example, using a TGA i1000 instrument (available from Instrument Specialists, Twin Lakes,Wis.). Heating rates of, for example, 5 or 7.5 K/min from roomtemperature to 900° C. were applied. A typical TGA plot of raw,unpurified SWCNT is shown in FIG. 2. FIG. 2 is a chart showing anexample of a thermogravimetric analysis of a raw carbon nanotubematerial containing about 50% of a catalyst metal impurity in accordancewith some embodiments of the present invention. This chart takes intoaccount the oxidation of initially present iron to iron oxide. FIG. 3 isa chart showing an example of a thermogravimetric analysis of a purifiedcarbon nanotube material in accordance with some embodiments of thepresent invention.

Analysis of multiple batches produced at identical process conditionsusing the disclosed subject matter led to very similar, nearlyoverlapping TGA plots, indicating a high degree of reproducibility ofpurity assessment. Quantification of the composition of a carbonaceousmaterial requires the knowledge of the composition of the metal phase inthe initial sample in order to account for increase of mass by oxidationof elemental iron.

Quantitative characterization of the metal phase can be conducted usingwide-angle X-Ray Diffraction (XRD) e.g., using a Rigaku RU300 X-raygenerator). Silicon (Si) can be added as an internal standard and XRDpatterns measured. Both maghemite (Fe₂O₃) and elemental iron (Fe) can beidentified in the as-produced material, whereas only some elemental iron(Fe) is typically found to remain in SWCNT that are purified accordingto the disclosed subject matter. Using an internal standard, analysis ofXRD spectra, allows for the quantitative determination of Fe-to-Fe₂O₃weight ratios. Assuming complete oxidation of elemental iron to Fe₂O₃during the TGA run, weight fractions for Fe, Fe₂O₃, and carbonaceousmaterial were determined. The trace metal contents of the purified CNTmaterial were analyzed by TGA employing the same procedures describedabove.

General Description of Ink Formation: Water-Based Ink

FIG. 4 is a process flow chart showing a method 400 for preparingaqueous-based carbon nanotube inks in accordance with some embodimentsof the present invention. The detailed flow chart shows the sequence ofunit operations in the processing of a water-based carbon nanotube inkformulation. It should be noted that, in the process flow chart of FIG.4 and other process flow charts described herein, some steps can beadded, some steps may be omitted, the order of the steps may bere-arranged, and/or some steps may be performed simultaneously.

As shown, as produced carbon nanotube raw material is heated on astir-hot plate in a mixture of hydrochloric acid (HCl) and hydrogenperoxide (H₂O₂) at 410. The concentration of hydrochloric acid cangenerally be between about 0.5N to about 10N and the concentration ofhydrogen peroxide can generally be between about 5% to about 30%. Theratio of HCl to H₂O₂ can be kept between about 3:1 to about 1:1. Itshould also be noted that the temperature at which stirring takes placecan generally be between about 50° C. to about 80° C.

It should be noted that as produced carbon nanotube raw material,purified carbon nanotube materials, fullerenes, and/or any otherfullerenic materials can be synthesized and/or processed by theapproaches described, for example, in Howard et al., U.S. Pat. No.5,273,729, filed May 24, 1991, Howard et al., U.S. Pat. No. 5,985,232,filed Sep. 11, 1996, Height et al., U.S. Pat. No. 7,335,344, filed Mar.14, 2003, Kronholm et al. U.S. Pat. No. 7,435,403, filed Jul. 3, 2003,and Howard et al., U.S. Pat. No. 7,396,520, filed Jan. 21, 2005, whichare hereby incorporated by reference herein in their entireties.

It should also be noted that the carbon nanotubes in these carbonnanotube-based inks can be synthesized such that any suitable percentageof a particular type of carbon nanotube is included in the carbonnanotube-based ink. For example, in some embodiments, at least 90% ofthe plurality of carbon nanotubes are single-walled carbon nanotubes. Inother embodiments, at least 90% of the plurality of carbon nanotubes aredouble-walled carbon nanotubes. Alternatively, at least 90% of theplurality of carbon nanotubes are multi-walled carbon nanotubes.

In one embodiment, the hydrogen peroxide can be slowly added to thecarbon nanotube acid mixture by, for example, using a syringe pump.Alternatively, the hydrogen peroxide needed for the reaction can begenerated in in-situ reactions.

The duration of the stirring/heating can range from about 1 hour toabout 48 hours. The CNT/acid/peroxide mixture on heating/stirring can bewashed with deionized (DI) water repeatedly in a filtration funnel orNutsche-type filter for large scale operations (see, e.g., 415-430 ofFIG. 4). The CNT-acid slurry is washed until it is neutral to pH paperand colorless.

The HCl/H₂O₂ slurry after repeated washings can be completely dried(e.g., less than 1 wt % of water), partially dried (e.g., less than 50wt % of water), or wet (e.g., the weight of CNT is less than the weightof water) at 435. Accordingly, the HCl/H₂O₂ slurry can be used in theform of a powder, partially dried paste, or a wet paste in downstreamapplications.

In some embodiments, the CNT powder or pastes as described above can bemixed with a non-ionic additive acting as a stabilizing agent at 440,such as diethylenetriamine (DETA), diisopropylethylamine (DIPEA orHunig's base), or triethanolamine. Other possible amines that can beused are, for example, ethylene diamine, aminoethyl ethanolamine,triethylene tetramine (TETA), tetraethylene pentamine (TEPA), andpentaethylene hexamine (PEHA). It should be noted that small molecularadditives can be used as stabilizing agents that disperse single-walledcarbon nanotubes without the elimination of the inter-band opticaltransitions.

In some embodiments, the CNT powder or pastes can also be mixed withcomposition of amines consisting one or more of the following amines:diethylenetriamine (DETA), diisopropylamine (DIPEA or Hunig's base),triethanolamine, ethylene diamine, aminoethyl ethanolamine, triethylenetetramine (TETA), tetraethylene pentamine (TEPA), and/or pentaethylenehexamine (PEHA) in different proportions.

Alternatively, the CNT powder or pastes can also be mixed withtriazole-based additives, such as 1,2,4-Triazole. For example,as-produced single-walled carbon nanotubes can be mixed with water andhydrochloric acid, where the concentration of the hydrochloric acid canbe about 37%. The mixture can be stirred, filtered, and rinsed, wherewater and nitric acid can be added to the rinsed mixture. The mixturecan then be heated and stirred, where the mixture can be filtered (e.g.,vacuum filtered through filter paper via, a Hirsch funnel) aftercooling. The purified single-walled carbon nanotubes can be rinsed withdeionized water until pH neutral to pH paper. The purified single-walledcarbon nanotubes in the form of a wet paste can be added to a solutionof 1,2,4-Triazole in deionized water.

It should be noted that, in some embodiments, the triazole-basedadditive can be unsubstituted 1,2,4-Triazole. For example, as shown inthe chemical formula below, each of R₁, R₂, and R₃ can be hydrogen.

Alternatively, in some embodiments, the triazole-based additive can besubstituted 1,2,4-Triazole. It should be noted that substituted1,2,4-Triazole can be used as an additive in water-based solvents andalso in non-aqueous solvents based on the selected substituents. Thatis, R₁, R₂, and R₃ can be selected in order to achieve greatersolubility in targeted or selected solvents.

For example, two or three substituting groups can be identical. In amore particular example, two or three substituting groups can beidentical and one or two groups can be hydrogen (—H). In yet anotherexample, R₁, R₂, and R₃ (sometimes referred to herein as “R”) can bestraight-chain or branched or cyclic alkyl chains (C₁ to C₂₀) which canbe unsubstituted, monosubstituted, or polysubstituted. Substituents canbe selected from at least one of the following: OH, OR, CO₂R, OOCR,SO₃H, X (where X is F, Cl, Br, NO₂ and/or CN), SO₂X, COX, NH₂, NR₂, NR₃⁺, substituted or unsubstituted benzyl (CH₂C₆H₅), substituted orunsubstituted phenyl, thiophene-radicals. H₂PO₄, and mixtures thereof.It should be noted that, in some embodiments, one or more CH₂ groups(including the one adjacent to triazole and establishing the link) canbe replaced by one of the following units: O, CO, NH, NHR⁺, SO₂, acyclic alkyl, a substituted or unsubstituted aromatic ring containingonly carbon or carbon and heteroatoms, the latter including nitrogen,sulfur, or oxygen.

Any suitable triazole-based additive can be used.

The mixture of CNT powder or pastes with the inclusion of one or more ofthe above-mentioned additives can then be agitated in deionized water,sonicated in deionized water, power sonicated in deionized water, ordispersed in water using a high-shear mixer at 445 and 450.

The CNT-water-stabilizer suspension or dispersion thus obtained can thenbe filtered through a coarse filter (e.g., a filter having, openingsgreater than about 10 micrometers) to eliminate larger suspendedparticles.

At 455, the resulting filtered solution can be centrifuged in anultracentrifuge that subjects a centrifugal force of greater than about5,000 g to about 200,000 g. The centrifuge used can be, for example, astatic batch rotor type centrifuge, a continuous flow type centrifuge,or a tubular flow type centrifuge. It should also be noted that volumesof CNT dispersions handled in a batch system can be about a few cc toseveral hundreds of cc. The volumes handled by the flow system can rangefrom about a few cc/minutes to several gallons/hr.

In some embodiments, a portion of the centrifuged mixture or solutioncan be collected for further processing, while the remaining portion ofthe centrifuged mixture or solution can be disposed or discarded. Forexample, upon sonicating and centrifuging the mixture, the toptwo-thirds of the centrifuged mixture can be collected for additionalprocessing to create a nanotube ink and the remaining one-third of thecentrifuged mixture can be disposed.

At 460, the centrifuged solution obtained can be filtered in atangential flow filtration assembly to remove extraneous carbonnanoparticles that are below a certain cut-off limit. It should be notedthat the filtration assembly can handle volumes ranging from about a fewcc/minutes to several gallons/hr.

A single-walled carbon nanotube (SWCNT) can be viewed as a rolled-upgraphene sheet within certain allowed chiralities. Based on thisgeometric, constraint, SWCNT produced by any method statistically ismade up of about one-third with metal-like electrical conduction andabout two-thirds showing a semiconducting behavior. Various chemical andelectrophoretic methods have demonstrated the separation of the carbonnanotubes by types, falling into the metallic and semiconducting ones.

In some embodiments, carbon nanotubes thus separated into metallic, andsemiconducting carbon nanotubes at various degrees of separation orenrichment can be made into the formation of carbon nanotube inksemploying a combination of steps described herein. For example, in someembodiments, the plurality of carbon nanotubes dispersed in the solventcan be separated such that at least 80% of the plurality of carbonnanotubes are semiconducting single-walled carbon nanotubes. In someembodiments, the plurality of carbon nanotubes can be enriched in eithersemiconducting or metallic single-walled carbon nanotubes, for example,in comparison to their initial abundance in the as-produced carbonnanotube materials (e.g., often approximately a 2:1 ratio ofsemiconducting vs. metallic corresponding to the ensemble of alltheoretically existing chiralities).

FIG. 5 is a chart showing an ultraviolet-visible absorption spectrum ofa carbon nanotube-based ink as a final product in a water-basedemploying DIPEA as the stabilizing agent. The interband opticaltransitions indicative of an intact electronic structure of the SWCNTare indicated with black arrows.

FIG. 6 is a chart showing an ultraviolet-visible absorption spectrum ofa carbon nanotube film deposited from the water-based ink on a 6″×4″glass substrate. The interband optical transitions arising from thefirst van Hove transitions are indicated with white arrows and thesecond van Hove transitions are indicated with black arrows.

FIG. 7 is a scanning electron microscope (SEM) image of a dense carbonnanotube network deposited on a sapphire substrate in accordance withsome embodiments of the present invention.

General Description of Ink Formation: Solvent-Based Ink

FIG. 8 is an exemplary schematic diagram showing the mechanisms forcreating a solvent-based ink. In some embodiments, the decomposablepolypropylene carbonate molecules wrap around the carbon nanotubes, asshown in FIG. 8, to help them stabilize in the chosen organic solvent.Alternatively, the polymer molecules may co-dissolve and function asviscosity adjustment agents, thereby aiding to control the rheologicalproperties of the CNT-solvent ink. In either embodiment, a CNT film thatis deposited on a solid substrate will still have the polymer molecules.The polymer molecules decompose on thermal annealing, in air into 100%non-toxic gaseous products. In addition, more than about 90% of thepolymer loss occurs below 200° C. at which temperature carbon nanotubesare very stable even in air. The resulting final product is a neatcarbon nanotube film.

For example, FIG. 9 is a chart showing a thermogravimetric analysis(TGA) plot of polypropylene carbonate (PPC) and the sharp decompositionof the polymer. As shown, more than 95% of the polymer loss occurs below200° C. at which temperature carbon nanotubes are very stable even inair. The derivative plot shows the sharp and rapid decomposition. Itshould be noted that approximately 0.5 wt % residue shown in FIG. 9arises from extraneous impurities.

Referring, to FIG. 10, FIG. 10 is a process flow chart showing one ofthe methods 1000 for preparing solvent-based carbon nanotube inks inaccordance with some embodiments of the present invention. The detailedflow chart shows the sequence of unit operations in the processing of asolvent-based carbon nanotube ink formulation. It should be noted that,in the process flow chart of FIG. 10 and other process flow chartsdescribed herein, some steps can be added, some steps may be omitted,the order of the steps may be re-arranged, and/or some steps may beperformed simultaneously.

Similar to FIG. 4, method 1000 begins with heating as produced carbonnanotube raw material on a stir-hot plate in a mixture of hydrochloricacid (HCl) and hydrogen peroxide (H₂O₂). The concentration ofhydrochloric acid can generally be between about 0.5N to about 10N andthe concentration of hydrogen peroxide can generally be between about 5%to about 30%. The ratio of HCl to H₂O₂ can be kept between about 3:1 toabout 1:1. It should also be noted that the temperature at whichstirring takes place can generally be between about 50° C. to about 80°C.

As described above, in one embodiment, the hydrogen peroxide can beslowly added to the carbon nanotube acid mixture by, for example, using,a syringe pump. Alternatively, the hydrogen peroxide needed for thereaction can be generated in in-situ reactions.

The duration of the stirring/heating can range from about 1 hour toabout 48 hours. The CNT/acid/peroxide mixture on heating/stirring can bewashed with deionized (DI) water repeatedly in a filtration funnel orNutsche-type filter for large scale operations (see, e.g., 115-130). TheCNT-acid slurry is washed until it is neutral to pH paper and colorless.

The HCl/H₂O₂ slurry after repeated washings can be completely dried(e.g., less than 1 wt % of water), partially dried (e.g., less than 50wt % of water), or wet (e.g., the weight of CNT is less than the weightof water). Accordingly, the HCl/H₂O₂ slurry can be used in the form of apowder, partially dried paste, or a wet paste in downstreamapplications.

As shown in FIG. 10, a solvent mixture is prepared by mixing an organicsolvent with a stabilizing additive at 1010. For example, a solventmixture can be prepared by dissolving an accurately weighed quantity ofa stabilizer in the range of about 0.1 to about 5 wt % of polypropylenecarbonate in a suitable organic solvent, preferablyN-methylpyrrolidinone (NMP).

Other suitable solvents that can also be used may include, for example,propylene glycol monomethyl ether acetate (PGMEA), cyclohexanone. Methylethyl ketone (MEK), methyl isopropyl ketone, etc.

It should be noted that polypropylene carbonates of different molecularweights can also be used as a viscosity adjusting agent instead of or inaddition to a stabilizing additive in solvents such asN-methylpyrrolidinone (NMP), propylene glycol monomethyl ether acetate(PGMEA), cyclohexanone, Methyl ethyl ketone (MEK), methyl isopropylketone, etc.

Referring back to FIG. 10, at 1020, as prepared raw carbon nanotubes,semi-purified carbon nanotubes, purified carbon nanotube powder, orpurified carbon nanotube pastes as described above can be mixed with thesolvent mixture at a concentration of about 0.1 to about 5 wt % and thenagitated, sonicated, power sonicated, or dispersed using a high-shearmixer.

The CNT-polymer-solvent suspension or dispersion can be filtered througha coarse filter (e.g., a lifter having openings greater than about 10micrometers) to eliminate larger suspended particles.

The resulting filtered solutions/dispersions can be centrifuged in anultracentrifuge that subjects a centrifugal force of greater than about5,000 g to about 200,000 g. The centrifuge used can be, for example, astatic batch rotor type centrifuge, a continuous flow type centrifuge,or a tubular flow type centrifuge. It should also be noted that volumesof CNT dispersions handled in a hatch system can be about a few cc toseveral hundreds of cc. The volumes handled by the flow system can rangefrom about a few cc/minutes to several gallons/hr.

It should be noted that the triazole-based additives used to aid in thedispersion and/or stabilization of the ink can be removed using anysuitable approach. Generally speaking, a film can be cast as astandalone film or deposited on a substrate using any suitable coatingtechnique, such as spin coating, spray coating, gravure coating, inkjetprinting, etc. A portion of the triazole-based removable additive can beremoved in the form of vapor or decomposed vapor by selecting thetemperature of deposition, which is generally between about 90° C. andabout 120° C. Further, the triazole-based additive can be removed byannealing the deposited film under vacuum, air or nitrogen, using, e.g.,suitable commercially available ovens. In the case of annealing underair, the temperature may not exceed about 200° C. In the case of vacuumor nitrogen flow ovens, the temperature can be as high as about 400° C.provided the stability and reactivity of the deposited substrate wouldallow. This ability to remove the triazole-based additive after filmformation provides many advantages in the formation of neat carbonnanotube networks allowing for enhanced performance as transparent,semi-transparent, non-transparent conductors or as part of a thin-filmtransistor.

EXAMPLES

The following examples further illustrate some embodiments of thepresent invention, but should not be construed as in any way limitingthe scope.

Example 1

Raw carbon nanotubes were produced in a combustion method employingmethane as the feed stock and iron nanoparticles formed in-situ by thedecomposition of iron pentacarbonyl. Approximately 970 mg of raw carbonnanotubes prepared by combustion of methane were added to 200 ml of DIwater in a 500 ml round bottom flask. To this mixture, 75 ml of 36%hydrochloric acid was added slowly with stirring followed by the slowaddition of 75 ml of ice cold, 30% hydrogen peroxide in drops. Thismixture was allowed to stir over a magnetic hot plate stirrer overnightat a temperature of about 60° C. The mixture was then allowed to cool toroom temperature without stirring. A black sediment of carbon nanotubessettled at the bottom while the supernatant liquid was deep yellow/brownand transparent due to the presence of iron ions. The supernatant liquidwas decanted into a larger flask. About 100 ml of DI water was added tothe solid contents, which was hand stirred and allowed, to settle overfew minutes. The new supernatant liquid turned pale and was decanted asdescribed before. This procedure was repeated until the supernatantliquid was colorless and clear. At this point the decanted liquid wasfiltered through a 90 mm diameter Whatman filter paper (450, hardened)in a ceramic Büchner funnel. This first stage wet CNT slurry collectedon the filter paper was washed until the washings were no different inpH compared to DI water as tested by a pH paper. A small portion of thiswet. CNT slurry was dried in the Büchner funnel by drawing air throughit by applying vacuum at the flask. The vacuum was applied with a simplerotary pump (10 mm of mercury) until the powder peeled off from thefilter paper and was collected in a bottle.

Example 2

A first stage wet CNT slurry prepared as described in Example 1 wastransferred back to a clean 500 ml round bottom (RB) flask from thefilter paper by washing with approximately 50 ml of DI water. To thisslurry, 100 ml of DI water and 50 ml of 6N nitric acid was added. Thenitric acid was added dropwise. The flask was fitted with a refluxcondenser cooled with running cold water and the mixture in the RB flaskwas stir heated on a hot plate to reflux. After refluxing for about 3hours, this mixture was allowed to cool to room temperature withoutstirring. A black sediment of carbon nanotubes settled at the bottom ofthe RB flask while the supernatant liquid was very pale yellow. Thesupernatant liquid was decanted into a larger flask. About 100 ml of DIwater was added to the solid contents in the RB flask and was handstirred before being allowed to settle over few minutes. The supernatantliquid in the RB flask turned colorless and clear and was decanted. Thedecanted liquid was filtered through a 90 mm diameter Whatman filterpaper (#50, hardened) in a ceramic Büchner funnel. The wet carbonnanotube slurry was washed until the washings were no different in pHcompared to DI water as tested by a pH paper. This resultant, secondstage wet CNT slurry was dried in the Büchner funnel by drawing airthrough it under vacuum. The vacuum was applied with a simple rotarypump (10 mm of mercury) until the powder peeled off from the filterpaper for collection in a bottle. Some of the second stage wet slurrywas only partially dried and stored as a purified CNT paste.

Example 3

In yet another purification method, as prepared raw carbon nanotubesamples were pre-washed in neutral DI water prior to acid treatment asdescribed below. An accurately weighed amount of raw carbon nanotubesless than about one gram) is placed in a thick wailed glass tube andsonicated with a power sonicator horn for 15 minutes. The resulting darksuspension was filtered through a cellulose filter (2-5 microns) for theremoval of finer particles. The CNT water paste collected over thefilter paper was used in the further purification process in the placeof as prepared CNT.

Example 4

In yet another acid purification process, approximately one gram of theCNT water paste collected as described in Example 3 was transferred to a2 liter round bottom flask and DI water was added to make up the volumeto 1000 ml. To this mixture, 100 ml of 36% HCl and 50 ml of 30% hydrogenperoxide were added and allowed to stir on a hot plate at roomtemperature overnight. The CNT acid slurry was washed with DI water asdescribed in Examples 1 and 2. The purified wet CNT slurry wastransferred to a 500 ml round bottom flask to which 100 ml of 6N nitricacid and 250 ml of DI water were added. The CNT acid mixture wasrefluxed for 3 hours and allowed to cool. The nitric acid water slurrywas filtered and washed through a filter paper and allowed to drypartially. The partially dried CNT was further used in the preparationof a water-based CNT ink as described herein.

Example 5

In yet another example, about 500 mg of the partially dried CNT paste asdescribed in Example 4 was placed in a ceramic cup to which 2 ml ofN,N-diisopropylethylamine (DIPEA) was added and mixed well manually witha ceramic paddle. The mixture was allowed to sit overnight. TheCNT-DIPEA paste was transferred to a 1 L conical flask to which 750 mlof DI water was added. The mixture was sonicated for about 1 hour in aBranson bath sonicator and allowed to stand for about an hour. It wasresonicated for one more hour and the resulting suspension wascentrifuged at 15,000 RPM at 10° C. for 1 hour. The supernatant liquidwas collected as a stable water-based CNT ink.

Example 6

In yet another acid purification process of the raw carbon nanotubes,two grams of the raw carbon nanotubes in a one liter round bottom flask,500 ml of DI water, 100 ml of 36% HCl and 100 ml of ice cold hydrogenperoxide were added and set for stirring at approximately 60° C.overnight. The CNT acid shiny was washed with DI water as described inExamples 1 and 2. The purified wet CNT slurry was transferred to a 500ml round bottom flask to which 100 ml of 6N nitric acid and 250 ml of DIwater were added. The CNT acid mixture was refluxed for 3 h and allowedto cool. The nitric acid water slurry was filtered through a #50 Whatmanfilter paper in a Buchner funnel and allowed to dry partially. Thepartially dried CNT as further used in the preparation of asolvent-based CNT ink as described below in Example 7.

Example 7

A solution of polypropylene carbonate (PPG) (a commercial sample fromNovomer Inc.) was prepared by dissolving PPG in 20 ml ofN-methylpyrrolidinone (NMP) to a concentration of 2 mg/mL. To thissolution, 20 mg of the partially dry CNT prepared as described inExample 6 was added and sonicated in a bath sonicator for 1 hour. Thesolution in the test tube was transferred to a conical flask. 80 ml ofNMP was added to the mixture to make up the total volume to 100 ml. Thesolution was sonicated 90 minutes and centrifuged at 10,000 RPM for 1how at 10° C. A very stable dark solvent-based CNT ink was obtained andbottled.

Example 8

In some embodiments, a triazole-based removable additive can be used informulating surfactant-free single-walled carbon nanotube inks.

In this example, as-produced single-walled carbon nanotubes were mixedwith 250 mL of deionized water and 50 mL of concentrated hydrochloricacid (HCl) (37%) in a round bottom flask. The flask was then connectedto an air cooling condenser/distillation column, and the mixture wasallowed to stir at medium speed overnight. The mixture was vacuumfiltered through a Whatman 50 filter paper placed in a Hirsch funnel.The filtered single-walled carbon nanotubes were rinsed with deionizedwater and transferred back into a round bottom flask to which 250 mL ofdeionized water and 100 mL of 6N nitric acid were added. The flask wasthen attached to a reflux condenser with circulated chilled water, wherethe mixture was heated to a boil on a hot plate, stirred at mediumspeed, and allowed to reflux for about three hours. The mixture wasallowed to cool and the contents were vacuum filtered through a Whatman50 filter paper via a Hirsch funnel. The resulting purified carbonnanotubes were rinsed with deionized water until pH neutral. Thepurified single-walled carbon nanotubes were collected as a wet paste.The purified single-walled carbon nanotubes can, in some embodiments, bestored in an amber glass vial to protect against direct lightillumination.

Example 9

In some embodiments, the purified single-walled carbon nanotubesprepared in Example 8 can be used to prepare a nanotube ink thatincludes a triazole-based additive.

In this example, a 0.1 wt % solution of 1,2,4-Triazole in deionizedwater was prepared. A particular amount of purified single-walled carbonnanotubes in the form of a wet paste, prepared as described in Example 8above, was added to the solution of 1,2,4-Triazole and deionized waterto make a solution with 0.1 wt % single-walled carbon nanotubes. Thismixture is shear milled for about 15 minutes at 11,000 RPM using a shearmill (e.g., using an IKA Ultra-Turrax T-25 shear mill). The mixture wasthen sonicated and centrifuged (e.g., at 5000 RPM for about 1 hour)using, for example, a 5210 Branson sonication bath and a ThermoScientific Jouan C3i Multifunction Centrifuge. In some embodiments, aportion of the centrifuged mixture was collected for further processing,while the remaining portion was discarded. In this example, the toptwo-thirds of the centrifuged mixture was collected for furtherprocessing into a nanotube ink, while the bottom one-third of thecentrifuged mixture was disposed. The collected centrifuged mixture wassonicated for about 1 hr in a sonication bath and the mixture was thenfiltered through a coarse filter to remove remaining clumps oraggregates.

The resulting mixture was then stored as a carbon nanotube ink.

FIG. 12 is a chart showing an ultraviolet-visible absorption spectrum ofa single-walled carbon nanotube ink prepared in accordance with Example9 that includes purified single-walled carbon nanotubes prepared inaccordance with Example 8 in accordance with some embodiments of thepresent invention. In the example of FIG. 12, theultraviolet-visible-near-infrared absorption spectrum of the carbonnanotube ink was measured from 300 nanometers to 1,100 nanometers usinga Shimadzu V3101 spectrophotometer to investigate the electronicstructure of the single-walled carbon nanotubes in the ink. It should benoted that FIG. 12 confirms the presence of single-walled carbonnanotubes in the nanotube ink.

Example 10

In some embodiments, an optional purification procedure can beperformed.

In this example, a particular amount of purified single-walled carbonnanotubes in the form of a wet paste, prepared as described in Example 8above, was added to 0.5N ammonium hydroxide (NH₄OH) in deionized water.This mixture was sonicated for about 30 minutes followed by vacuumfiltration through a Whatman 50 filter paper via a Hirsch funnel. Thesingle-walled carbon nanotubes were then washed with ammonium hydroxidefollowed by deionized water until pH neutral. The single-walled carbonnanotubes were collected for preparing a nanotube ink. The filtrate wasconcentrated in a rotary evaporator and the concentrated dispersion wasanalyzed by spectroscopy and electron microscopy. A transmissionelectron micrograph (TEM) image of the purified single-walled carbonnanotubes is shown in FIG. 13. More particularly, the TEM image of FIG.13 shows that the above-mentioned washing procedure removed amorphous,non-tubular carbon impurities.

Example 11

In some embodiments, a water-based nanotube ink can be formed withsingle-walled carbon nanotubes purified as described using the optionalpurification procedure of Example 10.

In this example, a 0.1 wt % 1,2,4-Triazole solution in deionized waterwas prepared. A particular amount of purified single-walled carbonnanotubes in the form of a wet paste, prepared with the purificationprocedure as described in Example 9 above, was added to make a solutionwith 0.1 wt % single-walled carbon nanotubes. This mixture was shearmilled for about 15 minutes at 11,000 RPM using a shear mill (e.g.,using an IKA Ultra-Turrax T-25 shear mill). The mixture was thensonicated and centrifuged (e.g., at 5000 RPM for about 1 hour) using,for example, a 5210 Branson sonication bath and a Thermo ScientificJouan Multifunction Centrifuge. In some embodiments, the top two-thirdsof the centrifuged mixture was collected for further processing, whilethe bottom one-third of the centrifuged mixture was disposed. Thecollected centrifuged mixture was sonicated for about 1 hr in asonication bath and the mixture was then filtered through a coarsefilter to remove remaining clumps or aggregates.

The resulting dispersion was then stored as a carbon nanotube ink.

FIG. 14 is a chart showing an ultraviolet-visible-near-infraredabsorption spectrum of a single-walled carbon nanotube ink prepared inaccordance with Example 11 that includes purified single-walled carbonnanotubes prepared in accordance with Example 10 in accordance with someembodiments of the present invention. In the example of FIG. 14, theultraviolet-visible-near-infrared absorption spectrum of the carbonnanotube ink was measured from 300 nanometers to 1,100 nanometers usinga Shimadzu V3101 spectrophotometer to investigate the electronicstructure of the SWCNT in the ink. As shown, the optical absorptionsarising from the interband electronic transitions in the single-walledcarbon nanotubes are clearly present when amorphous carbon impuritiesare removed or reduced using the purification procedure.

Applications

The resulting ink composition can be coated on any suitable substrate(e.g., a glass substrate, a plastic substrate, a sapphire substrate,etc.) using, a number of techniques, including inkjet printing, spincoating, spray coating, etc. In addition, the resulting ink compositioncan be used in numerous applications ranging from liquid crystaldisplays CDs), antistatic coatings, electrodes, touchscreens, andnumerous other applications.

For example, in one embodiment, an ink with extensive bundling, of SWCNTand, thus, limited suspendability can be used for applications, such asbattery electrodes or capacitors. In another embodiment, an ink withindividually suspended SWCNTs can be used for applications, such astransparent conductive coatings.

In another suitable embodiment, an ink prepared as described herein canbe coated onto plastic beads ranging in size from about 10 nanometers toseveral hundred micrometers (μm). Alternatively or additionally, the inkcan be coated onto plastic fibers, glass fibers, or ceramic fibershaving diameters ranging from 10 nanometers to several hundredmicrometers (μm) and having aspect ratios ranging from 10 to 10⁶. In amore particular example, a scanning electronic microscope (SEM) image ofplastic beads coated with carbon nanotubes using the above-described inkis shown in FIG. 11.

It should be noted that the coating of plastic, glass, ceramic, and/orother suitable substrates and materials can be used to enhance electricand/or thermal conductivity. Accordingly, this can be used forelectrically conducting films or electrostatic dissipation applications.

It should further be noted that the inks prepared as described hereincan be used as a medium for chemical functionalization of carbonnanotubes with, for example, but not limited to, reactions withdiazonium salts, Diels-Alder reagents, cycloadditions, halogenations,nucleophilic or radical additions (see, e.g., Tasis et al., Chem. Rev,2006, 106, 1105-1136; and Zhang et al., J. Am. Chem. Soc. 2009, 131,8446-8454). Such functionalization may selectively occur with metallicor semi-conducting carbon nanotubes.

Mixtures between functionalized and unfunctionalized carbon nanotubesdispersed in the above-mentioned inks can be separated by densitygradient centrifugation or electrophoresis.

Carbon nanotubes that are functionalized in other reaction media, suchas organic solvents (e.g., o-dichlorobenzene, tetrahydrofuran (THF),etc.) or water in the presence or not of ionic surfactants, areredissolved in the above-mentioned inks.

Accordingly, solvent-based and water-based carbon nanotube inks withremovable additives are provided.

Although the invention has been described and illustrated in theforegoing illustrative embodiments, it is understood that the presentdisclosure has been made only by way of example, and that numerouschanges in the details of implementation of the invention can be madewithout departing from the spirit and scope of the invention. Featuresof the disclosed embodiments can be combined and rearranged in variousways.

What is claimed is:
 1. An ink composition comprising: a plurality ofcarbon nanotubes; a solvent; and a triazole-based removable additivethat stabilizes the plurality of carbon nanotubes in the solvent,wherein the triazole-based removable additive is a 1,2,4-triazolecompound having a chemical formula:

wherein each of R₁, R₂, and R₃ is hydrogen.
 2. The ink composition ofclaim 1 wherein the plurality of carbon nanotubes comprise one or moreof: single-walled carbon nanotubes, metallic single-walled carbonnanotubes, semiconducting single-walled carbon nanotubes, and mixturesthereof.
 3. The ink composition of claim 1, wherein the plurality ofcarbon nanotubes are enriched in one of: metallic single-walled carbonnanotubes and semiconducting single-walled carbon nanotubes.
 4. The inkcomposition of claim 1, wherein the solvent is one of: water,N-methylpyrrolidinone (NMP), propylene glycol monomethyl ether acetate(PGMEA), methyl ethyl ketone (MEK), and methyl isopropyl ketone.
 5. Theink composition of claim 1, wherein the triazole-based removableadditive is selected to act as a dispersal agent and a stabilizationagent.
 6. The ink composition of claim 1, wherein the ink composition isin the form of a film deposited on a substrate and wherein a substantialportion of the triazole-based removable additive is removed from thefilm by thermal annealing.
 7. The ink composition of claim 1, whereinthe additive is a non-ionic additive.
 8. A method of preparing an inkcomposition according to claim 1, the method comprising: mixing theplurality of carbon nanotubes, the triazole-based removable additive,and the solvent, wherein the plurality of carbon nanotubes are dispersedwithin the solvent and wherein the triazole-based removable additivestabilizes the plurality of carbon nanotubes that are dispersed in thesolvent.
 9. The method of claim 8, wherein the plurality of carbonnanotubes comprise one or more of: single-walled carbon nanotubes,metallic single-walled carbon nanotubes, semiconducting single-walledcarbon nanotubes, and mixtures thereof.
 10. The method of claim 8,wherein the plurality of carbon nanotubes are enriched in one of:metallic single-walled carbon nanotubes and semiconducting single-walledcarbon nanotubes.
 11. The method of claim 8, wherein the solvent is oneof: water, N-methylpyrrolidinone (NMP), propylene glycol monomethylether acetate (PGMEA), methyl ethyl ketone (MEK), and methyl isopropylketone.
 12. The method of claim 8, further comprising providing theplurality of carbon nanotubes in the form of a wet paste to a solutionthat includes the triazole-based removable additive and the solvent. 13.The method of claim 8, further comprising stabilizing the plurality ofcarbon nanotubes by providing the triazole-based removable additive to asolution that includes the plurality of carbon nanotubes and thesolvent.
 14. The method of claim 8, further comprising: stabilizing theplurality of carbon nanotubes by applying the triazole-based removableadditive to the plurality of carbon nanotubes prior to dispersing theplurality of carbon nanotubes in the solvent; and providing the solventto the plurality of carbon nanotubes and the triazole-based removableadditive.
 15. The method of claim 8, further comprising: applying theink composition to a substrate; and removing a substantial portion ofthe triazole-based removable additive by thermal annealing, wherein thetriazole-based removable additive is removed after applying the inkcomposition to a substrate.
 16. The method of claim 8, furthercomprising purifying the plurality of carbon nanotubes prior to addingthe triazole-based removable additive and the water-based solvent. 17.The method of claim 16, wherein the plurality of carbon nanotubes arepurified by washing the plurality of carbon nanotubes in a solution ofammonium hydroxide.
 18. The method of claim 8, further comprisingpurifying a mixture including the plurality of carbon nanotubes, thetriazole-based removable additive, and the water-based solvent byreducing impurities using centrifugation.
 19. The method of claim 18,wherein the centrifugation reduces amorphous carbon impurities.
 20. Themethod of claim 18, wherein a first portion of the centrifuged mixtureis stored as the ink composition and a second portion of the centrifugedmixture is discarded.
 21. The method of claim 18, further comprisingpassing at least a portion of the centrifuged mixture through a filterto remove particle impurities having a diameter greater than a givensize.
 22. A method of preparing an ink composition according to claim 1,the method comprising: providing a paste that includes the plurality ofcarbon nanotubes which are single-walled carbon nanotubes; purifying thepaste that includes the plurality of single-walled carbon nanotubes in asolution of ammonium hydroxide to substantially reduce amorphous carbonimpurities; forming a mixture by adding the 1,2,4-triazole compoundhaving the chemical formula:

wherein each of R₁, R₂, and R₃ is hydrogen, and the solvent which is awater-based solvent to the purified paste that includes the plurality ofsingle-walled carbon nanotubes, wherein the plurality of single-walledcarbon nanotubes are dispersed within the water-based solvent andwherein the 1,2,4-triazole compound stabilizes the plurality ofsingle-walled carbon nanotubes that are dispersed in the water-basedsolvent; and purifying the mixture by centrifugation, wherein a firstportion of the centrifuged mixture is stored as the ink compositionaccording to claim 1 and a second portion of the centrifuged mixture isdiscarded.